The application claims benefit of Japanese Application No. 2001-361096 filed in Japan on Nov. 27, 2001, the contents of which are incorporated by this reference.
The present invention relates to a macro lens and a camera comprising the same. The present invention is particularly suitable for silver-halide or digital cameras. More specifically, the present invention is directed to a macro lens suitable for use as an interchangeable lens applicable to silver-halide or digital single-lens reflex cameras.
So far, many macro lenses have been proposed as interchangeable lenses for single-lens reflex cameras or digital single-lens reflex cameras.
For focusing macro lenses, floating techniques involving the movement of a plurality of groups have been used, because of a large fluctuation of spherical aberrations between at infinity and at close range.
Conventional macro lenses, for the most part, give weight to designs having a magnification of about {fraction (1/10)} while they are well balanced at infinity and at close range, and so their phototaking performance at infinity is inferior to that of general lens systems that are in no sense any macro lens systems.
Many macro lenses have large fluctuations of spherical aberrations and field curvature at infinity to close range upon focusing, and so such aberrations are reduced by means of floating.
With large-aperture macro lenses, however, it is difficult to control these fluctuations, and so field curvature and coma in particular become noticeable in short range regions.
In view of such problems with the prior art as explained above, one object of the present invention is to provide a fast macro lens that is well corrected for aberrations even upon close-range shooting, and a camera comprising the same.
Another object of the present invention is to provide a large-aperture macro lens that has reduced aberration fluctuations at every range from infinity to close range and an F-number of about 1.8.
Yet another object of the present invention is to provide a macro lens best suited for half film size and an image circle nearly half the diagonally 135-long format.
According to the first aspect of the present invention, these objects are achievable by the provision of a macro lens, characterized by comprising, in order from its object side, a first lens group having positive power and a second lens group having positive power, wherein in the first lens group a negative meniscus lens component concave on its object side is located nearest to the object side of the first lens group, and upon focusing from an object point at infinity to the closest object point, the first lens group and the second lens group move independently toward the object side of the macro lens while the spacing between them varies.
According to the second aspect of the present invention, there is provided a macro lens characterized by comprising, in order from an object side thereof, a first lens group having positive power, a second lens group having negative power and a third lens group having positive power, wherein in the first lens group a negative meniscus lens component concave on its object side is located nearest to the object side of the first lens group, and upon focusing from an object point at infinity to the closest object point, the lens groups move independently toward the object side of the macro lens while a spacing between adjacent lens group varies.
In what follows, why the aforesaid arrangements are used, and how they work will be explained.
The macro lens of the present invention may be used on cameras in general. In particular, this macro lens is best suited for use on a single-lens reflex camera (irrespective of whether or not lens replacement is needed), which must take a back focus enough to mount therein an observation optical path-dividing mechanism. The macro lens of the present invention is also applicable to just only silver-halide silver cameras but also to other cameras using electronic image pickup devices such as solid-state image pickup devices or CCDs.
Comprising two positive lens groups, the micro lens of the present invention makes correction for aberrations upon focused at close range by means of the floating action ensuing from independent movement of them.
For lens replacement, a macro lens for single-lens reflex cameras must take a given back focus length. Although the lens of the present invention is a medium-telephoto lens system having a view angle of 2xcfx89≈about 24xc2x0, its focal length with respect the view angle reduces by nearly half that of the 135 format. With the lens system according to the specification of the present invention, it is thus not easy to make sure of the back focus.
To ensure this back focus, the first lens component located nearest to the object side of the first lens group having positive power must be formed of a negative lens component. To bring the principal point at a position in the rear of the lens system and on the image side of the lens system, the first lens component should preferably be defined by a negative meniscus lens component concave on its object side. With this arrangement, it is possible to take an adequate back focus length and, hence, make sure of space large enough to receive a quick-return mirror.
Thus, the first micro lens of the present invention has the structure comprising, in order from an object side thereof, a first lens group having positive power and a second lens group having positive power, wherein in the first lens group a negative meniscus lens component concave on an object side thereof is located nearest to the object side of the first lens group, and upon focusing from an object point at infinity to the closest object point, the first lens group and the second lens group move independently toward the object side of the macro lens while the spacing between them varies.
Alternatively, the second macro lens of the present invention has the structure comprising, in order from an object side thereof, a first lens group having positive power, a second lens group having negative power and a third lens group having positive power, wherein in the first lens group a negative meniscus lens component concave on an object side thereof is located nearest to an object side of the first lens group, and upon focusing from an object point at infinity to the closest object point, the lens groups move independently toward the object side of the macro lens while the spacing between adjacent lens group varies.
In this embodiment of the present invention, the three lens groups are positioned in a nearly symmetric power profile of +-+, and aberrations upon focusing at close range are corrected by the floating action ensuing from their independent movement.
To hold back aberration fluctuations every range from infinity to close range while close-range performance is improved, floating should preferably be carried out by the movement of the three lens groups of +-+ power profile. With this arrangement, fluctuations of spherical aberrations and coma with focusing can be reduced as much as possible, and field curvature fluctuations can be easily corrected as well.
It is here understood that the two-group arrangement is overwhelmingly favorable for the associated lens barrel structure, and so can be much more reduced in terms of performance variations by fabrication errors than the three-group arrangement.
A stop should preferably be inserted in the first lens group in the case of the two-group arrangement, and in the second lens group in the case of the three-group arrangement.
Where the height of a marginal ray minimizes is in the first lens group in the case of the two-group arrangement, and in the second lens group in the case of the three-group arrangement; it is most preferable to locate the stop at that position, at which the stop can be made more compact because its diameter can become short. At that position, the marginal ray minimizes in height, taking the form of a substantially afocal ray, so that even when the stop displaces to and fro due to stop-mounting position misalignments, etc., there is no or little noticeable inconvenience.
It is also desired to locate a plurality of positive lens components between the negative meniscus lens component and the stop.
For the first lens group, it is preferable that at least two positive lens components are located after the first lens component. To correct the first lens group for spherical aberrations, positive refracting power is required; to make better correction for them, however, at least two positive lens components are necessary. With this arrangement, it is also possible to avoid making the diameter of the stop located in the rear of these lens components larger than required.
It is desired that the lens components positioned just before and just after the stop be formed of negative lens components.
When the negative lens components are disposed before and after the stop, a relatively symmetric arrangement is obtained with respect to the stop. This arrangement is also favorable for correction of distortions.
The first lens group in the case of the two-group arrangement, and the combined first and second lens group in the case of the three-lens arrangement comprises, in order from an object side thereof, a negative meniscus lens component concave on its object side, a positive lens group, a positive lens component in which the object-side surface thereof is smaller in terms of the absolute value of the radius of curvature than the image-side surface thereof, a negative lens component in which the image-side surface thereof is smaller in terms of the absolute value of the radius of curvature than the object-side surface thereof, a stop, a negative lens component in which the object-side surface thereof is smaller in terms of the absolute value of the radius of curvature than the image-side surface thereof, and a positive lens component in which the image-side surface thereof is smaller in terms of the absolute value of the radius of curvature than the object-side surface thereof.
This arrangement is of the so-called Gauss type. To enter light from the first lens component into the subsequent negative lens component while it is converged little by little, the positive lens group and the positive lens component in which its object-side surface is smaller in terms of the absolute value of the radius of curvature than its image-side surface are provided.
In this arrangement, aberrations are corrected by an air lens having strong negative power, which is defined by the two lens components with the stop interposed between them.
The subsequent positive lens component serves to prevent the diameter of a light beam from becoming large while the symmetry of the Gauss type optical system is maintained and the angle of incidence of light on the subsequent second lens group (the third lens group in the case of the three-group arrangement) is controlled.
It is noted that the lens component used may be either a single lens component or a cemented lens component. Although it is acceptable to cement adjacent lens components together, it is understood that aberrations can be well corrected by use of the Gauss type, and so it is preferable to construct all lens components other than those in the final lens group (all lens components in the first lens group in the case of the two-group arrangement, and in the first and second lens groups in the case of the three-group arrangement) of single lenses, thereby achieving cost reductions.
It is also preferable that the second lens group in the case of the two-group arrangement, and the third lens group in the case of the three-group arrangement comprise a positive doublet component where positive and negative lens components are cemented together.
The final lens group (the second lens group in the case of the two-group arrangement, and the third lens group in the case of the three-group arrangement) should preferably be constructed of a reduced number of lens components for the purpose of making the length of the lens group short. More preferably, however, the positive doublet component should be used because correction of aberrations can be made with such a reduced number of lens components.
When an image is formed on the light-receptive surface of an electronic image pickup device, it is required to diminish the angle of incidence of an off-axis chief ray on that light-receptive surface. This also makes some contribution to correction of chromatic aberration of magnification.
To make satisfactory correction for chromatic aberrations, it is desired to use at least one positive lens component and at least one negative lens component in the final lens group. To be more effective, these lens components should be cemented together.
Several conditions preferable for the aforesaid arrangements or embodiments are now explained.
Preferably, the focal length of the first lens should comply with the following condition (1):
xe2x88x924 less than fF/fL less than xe2x88x921xe2x80x83xe2x80x83(1)
where fF is the focal length of the negative meniscus lens component located nearest to the object side of the macro lens, and fL is the focal length of the macro lens upon focused on an object point at infinity.
As the upper limit of xe2x88x921 to condition (1) is exceeded, the power of this lens becomes too strong and every aberration from spherical aberration to come to field curvature becomes too large to be corrected at other lenses. At less than the lower limit of xe2x88x924, it is difficult to make sure of any adequate back focus because the power of the first lens becomes weak.
If the upper and the lower limit are defined as mentioned below, it is then possible to make the aforesaid effects much more satisfactory.
xe2x88x922.5 less than fF/fL less than xe2x88x921.8xe2x80x83xe2x80x83(1)xe2x80x2
The first lens should also preferably comply with the following condition (2):
xe2x88x92-12.5 less than (r1+r2)/(r1xe2x88x92r2) less than xe2x88x920.85xe2x80x83xe2x80x83(2)
where r1 is the radius of curvature of the object-side surface of the negative meniscus lens located nearest to the object side of the macro lens, and r2 is the radius of curvature of the image-side surface of the negative meniscus lens located nearest to the object side of the macro lens.
As the lower limit of xe2x88x9212.5 to condition (2) is not reached, the negative refracting power of the first lens becomes weak, and so it is difficult to make sure of any back focus as is the case with condition (1). Exceeding the upper limit of xe2x88x920.85 to condition (2) is not preferred because the negative refracting power of the first surface becomes too strong to cause noticeable fluctuations of spherical aberrations at every range from infinity to the closest object point.
More preferably in this case, the lower limit to condition (2) should be defined as given below.
xe2x88x928.5 less than (r1+r2)/(r1xe2x88x92r2) less than xe2x88x920.85xe2x80x83xe2x80x83(2)xe2x80x2
When a large-aperture lens system has an F-number of up to 1.8, it is difficult to make correction for spherical aberrations and coma. To ensure a large-aperture F-number in the case of the two-group arrangement, it is thus preferable to limit the focal length of the first lens group within the following range:
0.5 less than f1/fL less than 1.8xe2x80x83xe2x80x83(3-1)
where f1 is the focal length of the first lens group, and fL is the focal length of the macro lens upon focused on an object point at infinity.
Likewise in the three-group arrangement, it is preferable to limit the focal length of the first lens group within the following range:
0.5 less than f1/fL less than 1.8xe2x80x83xe2x80x83(3-2)
where f1 is the focal length of the first lens group, f3 is the focal length of the third lens group, and fL is the focal length of the macro lens upon focused on an object point at infinity.
As the lower limit of 0.5 to condition (3-1) or (3-2) is not reached or the power of the first lens group becomes strong, an axial marginal ray is largely refracted at a fast F-number, and so it is difficult to make correction for spherical aberrations on an object point at infinity. As the upper limit of 1.8 to these conditions is exceeded, the lens system becomes large.
To make better correction for spherical aberrations on an object point at infinity, it is preferable to make the refracting power of the first lens group in the case of two-group arrangement weaker or to comply with condition (3-1)xe2x80x2, and the refracting power of the first lens group in the case of the three-group arrangement stronger or comply with condition (3-2)xe2x80x2.
1.0 less than f1/fL less than 1.8xe2x80x83xe2x80x83(3-1)xe2x80x2
0.5 less than f1/fL less than 1.0xe2x80x83xe2x80x83(3-2)xe2x80x2
At greater than the lower limit of 1.0 or 0.5 to these conditions, it would be possible to achieve a faster lens system of improved performance.
In the case of the two-group arrangement, focusing is well achievable by floating ensuing from the movement of the respective lens groups. However, the bending of spherical aberrations at close range tends to become large. To minimize this, it is desired to limit the focal length of the lens group located nearest to the image side of the macro lens within the range defined by the following condition (4-1):
1.8 less than f2/fL less than 3.5xe2x80x83xe2x80x83(4-1)
where f2 is the focal length of the second lens group, and fL is the focal length of the macro lens upon focused on an object point at infinity.
Likewise in the three-group arrangement, it is desired to limit the focal length of the third lens group within the range defined by the following condition (4-2):
1.8 less than f3/fL less than 3.5xe2x80x83xe2x80x83(4-2)
where f3 is the focal length of the third lens group, and fL is the focal length of the macro lens upon focused on an object point at infinity.
As the lower limit of 1.8 to these conditions is not reached, there is grave deterioration in performance such as spherical aberrations and field curvature at close range. Exceeding the upper limit of 3.5 to these conditions is not preferable because there is an increase in the quantity of movement of the second or third lens group upon focusing.
More preferably, conditions (4-1) and (4-2) should be reduced down as given below.
2.2 less than f2/fL less than 3.0xe2x80x83xe2x80x83(4-1)xe2x80x2
2.2 less than f3/fL less than 3.0xe2x80x83xe2x80x83(4-2)xe2x80x2
Inasmuch as the range defined by these conditions is satisfied, it would be possible to achieve the aforesaid effects in a more favorable manner.
By determining the focal lengths of the respective lens groups as explained above, the performance of the macro lens at close range can be fully achieved even when it is designed in such a way that its performance at infinity is on the same level as that of ordinary lenses.
In view of a large-aperture macro lens, the macro lens of the present invention should preferably comply with the following conditions (5), (6) and (7):
xe2x88x921.0 less than MG less than xe2x88x920.4xe2x80x83xe2x80x83(5)
7xc2x0 less than SW less than 16xc2x0xe2x80x83xe2x80x83(6)
1.0 less than F less than 3.0xe2x80x83xe2x80x83(7)
Here MG is the maximum magnification, SW is the half view angle of incidence of a diagonal ray on the maximum image height in the image pickup range of a camera body upon focused at infinity, provided that when the image pickup range of the image pickup surface is arbitrarily variable, SW is the maximum value in the possible widest range, and F is the F-number of the macro lens upon focused on an object point at infinity and upon stop in.
The macro lens of the present invention should preferably have a maximum magnification conforming to condition (5), and should have an upper-limit magnification of about xe2x88x920.4. To achieve a magnification that is less than the lower limit of xe2x88x921.0, it is required to increase the number of lens components or make the F-number large.
As the upper limit of 16xc2x0 to condition (6) is so exceeded that the object range spreads, it is difficult to take photographs at an increased magnification unless the camera is as close to subjects as possible. Insofar as the range of this condition is satisfied, it would be easy to take photographs at an increased magnification while the camera is relatively close to subjects. As the lower limit of 7xc2x0 is not reached, the focal length of the macro lens becomes long and so its total length becomes long; it is difficult to slim down the macro lens system.
A lens system departing from the range of condition (7) can no longer be referred to as a large-aperture lens. To meet condition (7) in particular, it is preferable to use an anomalous dispersion glass.
If the upper limit of 3.0 to condition (7) is down to 2.0 as mentioned below, it is then possible to obtain a faster lens system. This is particularly true for a large-aperture lens.
1.0 less than F less than 2.0xe2x80x83xe2x80x83(7)xe2x80x2
With a lens system having a magnification of about 0.5 like one contemplated herein, there is noticeable deterioration in its performance due to chromatic aberrations, although aberrations can be corrected at the design reference wavelength. According to the present invention wherein an anomalous dispersion glass is used in the rear of the stop, a large-aperture lens having an increased magnification can be achieved while corrected for longitudinal chromatic aberration and chromatic aberration of magnification.
As already explained, focusing is carried out by independent movement of the respective lens groups. In this case, however, the quantity of movement of the first lens group should preferably comply with the range defined below.
0.4 less than xcex94d1/fL less than 0.8xe2x80x83xe2x80x83(8)
Here xcex94d1 is the quantity of movement of the first lens group upon focused from an object point at infinity to the closest object point, and fL is the focal length of the macro lens upon focused on an object point at infinity.
When the power of the first lens group is within the range defined by conditions (3-1) and (3-2), the quantity of movement of the first lens group must be greater than the lower limit of 0.4 to condition (8). Below that lower limit, it is impossible to carry out phototaking within the range defined by the upper limit of condition (5) or the macro lens fails to serve its own function. As the upper limit of 0.8 to condition (8) is exceeded, it may be possible to obtain high enough magnification for macro-photography; however, the quantity of movement of the first lens group becomes unacceptably large for mechanical construction.
Further, the lens system of the present invention should preferably comply with the following conditions (9) and (10).
13 mm greater than IH greater than 10 mmxe2x80x83xe2x80x83(9)
3.5 greater than fb/IH less than 2.8xe2x80x83xe2x80x83(10)
Here IH is the radius of an image circle upon focused on an object point at infinity, and fb is the back focus of the macro lens system upon focused on an object point at infinity.
These conditions are to determine the space necessary for the location therein of a quick-return mirror, etc. Condition (9) determines the radius of the image circle that is herein assumed. The dimensions necessary for ensuring space on which the mirror is placed in view of layout are within the range of condition (10). As the lower limit of 2.8 to condition (10) is not reached, the mirror space becomes insufficient, and exceeding the upper limit of 3.5 to condition (10) is not preferable because the camera body becomes too large.
Furthermore, the macro lens system of the present invention should preferably comply with the following condition (11).
1xc2x0 less than |EW| less than 11xc2x0xe2x80x83xe2x80x83(11)
Here EW is the angle with an optical axis of an emergent ray from a diagonal chief ray incident at the maximum image height on the image pickup surface of a camera body upon focused on an object point at infinity, provided that when the image pickup range of the image pickup surface is arbitrarily variable, EW is a value found at a position where the image height maximizes in the possible widest range.
The macro lens system of the present invention may be applied to digital cameras. In this case, however, the angle of incidence of light on an image pickup device such as a CCD becomes a problem. As the angle of incidence of light on the CCD or the like is too large, insufficient light quantity due to oblique incidence becomes a matter of concern. Especially when the image height increases, the exit angle of the macro lens system increases, resulting in increased rim ray attenuation at the CCD or the like. To minimize light quantity reductions due to this rim-ray attenuation, it is required to comply with condition (11). Condition (11) defines the angle of the emergent ray from a diagonal chief ray with the optical axis, i.e., the absolute value of the exit angle of the diagonal chief ray. A CCD or the like, when used with the macro lens system of the present invention, should have its oblique incidence properties in coincidence with those of the macro lens system. To keep the rim-ray attenuation due to oblique incidence of light on the CCD or the like at practically acceptable levels, it is desired that the angle of incidence of the diagonal chief ray on the CCD or the like, i.e., the exit angle of the optical system does not depart from the range of condition (11).
The micro lens of the present invention may be used on silver-halide film cameras as well as on cameras using electronic image pickup devices such as solid-state image pickup devices or CCDs. It is also possible to rely on a mount (of the screw or bayonet type, for instance) in such a way as to make the macro lens attachable to or detachable from a camera body. Preferably in this case, the half view angle of incidence of diagonal light rays should comply with the aforesaid condition (6).
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.